Choroidal melanoma is the most common primary ocular cancer among the adult population. It is an important disease, because treatment may destroy vision in the affected eye and the tumor can metastasize. Its treatment is very controversial and is currently the subject of a major NEI clinical trial. Enucleation is a common treatment, but there has only been recent emphasis to develop eye-sparing therapies. Unfortunately, these newer treatments have been only marginally successful in eradicating the tumor while sparing vision. The ultimate goal should be to improve therapy so that safer, more efficacious treatments can be applied sooner in the course of the disease. The ability to treat sooner and more aggressively may ultimately enhance patient survival. Some eye-sparing therapies (e.g., radiation therapy, photodynamic therapy, and immunotherapy) are dependent on oxygen, yet virtually nothing is known about the physiology of this tumor, including mechanisms for blood flow control. In other areas of cancer research, there has been recent interest in the modulation of nitric oxide (NO) to modify tumor blood flow and oxygenation to improve treatment. One major drawback of these strategies, however, is that NO modulators must typically be given systemically, and side effects may limit their clinical application. Since choroidal melanomas are often accessible to treatment using an episcleral approach, the utilization of compounds associated with NO to locally manipulate the microenvironment of this tumor is especially appealing, since systemic treatment can be avoided. The purpose of the proposed study is to test hypotheses relating to the effects of NO on blood flow and oxygenation in human choroidal melanoma growing in the choroid of the athymic rat. The hypotheses to be tested are that (1) NO synthetase (NOS) is expressed in human choroidal melanoma parenchyma and/or vasculature, (2) NO plays a role in regulating microvascular blood flow in human choroidal melanoma and (3) human choroidal melanoma in vivo has areas in which the oxygen tension is low (hypoxia) and the oxygenation of the tumor is dependent on NO availability. The results of these studies could lead to development and testing of new therapies for choroidal melanoma, based upon selective, local delivery of agents which modulate NO availability and modify tumor blood oxygenation, thereby enhancing the effectiveness of treatments directed toward salvage of the affected eye.